1. Field of the Invention
This invention relates to a method of treating exhaust gases containing sulfur dioxide and nitrogen oxides including nitrogen monoxide, nitrogen dioxide and the like. More particularly, it relates to a wet method of simultaneously removing sulfur dioxide and nitrogen oxides by contacting an exhaust gas containing sulfur dioxide and nitrogen oxides with an aqueous absorbing solution containing an iron chelate complex and an alkali metal sulfite and converting alkali metal sulfur oxides comprising an alkali metal dithionate formed and accumulated in the aqueous absorbing solution into useful sources.
2. Description of the Prior Art
The sulfur dioxide and nitrogen oxides which are contained in combustion exhaust gases discharged from industrial combustion installations such as boilers are pollutants causing air pollution, and it is an important problem to remove these pollutants. Many methods of removing these poison gases have been proposed. For example, U.S. Pat. No. 3,984,522 describes the use of an aqueous solution containing an iron chelate complex and an alkali metal sulfite as an absorbing solution in a wet method of removing sulfur dioxide and nitrogen oxides simultaneously. However, in industrially carrying out this method, an alkali metal dithionate are by-produced in the absorbing solution when sulfur dioxide and nitrogen oxides are removed from exhaust gases by absorption and accordingly, it is required to efficiently and economically treat and recover, as a useful source, the alkali metal dithionate. According to U.S. Pat. No. 4,044,101, the alkali metal dithionate which is recovered from the absorbing solution by crystallization is thermally decomposed to form an alkali sulfate and sulfur dioxide from which an alkali metal sulfite and gypsum are recovered. The alkali metal sulfite recovered can be reused as an absorbing agent, but the by-produced gypsum is hardly considered as a useful source.
Also it is known that the alkali metal sulfate, for example, sodium sulfate formed by the thermal decomposition of the alkali metal dithionate is reduced with carbon to sodium sulfide in connection with a kraft pulping process.